This invention relates generally to gray tone reproduction and has particular application to gray tone reproduction utilizing an ink jet printer of the general type described in Sweet et al U.S. Pat. No. 3,373,437. As disclosed in the Sweet et al patent, gray tones may be reproduced by controlling the rate at which drops are deposited upon a moving web. Behane et al U.S. Pat. No. 3,604,846 teaches an improved gray scale reproduction technique wherein drops are deposited at various positions within a two dimensional matrix in accordance with the gray level to be reproduced. Variations on the matrix approach are disclosed in Berry et al U.S. Pat. No. 3,977,007 and in Wong U.S. Pat. No. 4,032,978.
This invention applies more particularly to an ink jet printer of the type taught by Paranjpe et al in U.S. Pat. No. 4,112,469. In printers of that type there is provided a print head having a row of spaced jets which are scanned across a recording member in the form of a sheet mounted upon a rotating drum. The scanning is carried out by transporting the print head axially along the length of the drum in such a manner as to cause spiral interlacing of the printed tracks produced upon the sheet by the different jets. As taught by Paranjpe et al, spiral interlacing may be achieved by providing the print head with any convenient number of jet producing nozzles and separating those nozzles by an appropraite integral number of printing track widths. An "appropriate" integral number is any number which has no factor other than 1 as a common factor with the number of nozzles. As also taught by Paranjpe, the print head should be axially advanced at a speed such that during one rotation of the drum, the axial advance is equal to the width of one printed track, multiplied by a number equal to the number of nozzles. Printing control is accomplished by simultaneously scanning a document, which is positioned upon a document plane, and sweeping an image of the document past a line of photocells arranged in correspondence with the arrangement of jet printing nozzles. Each photocell is connected to switch an associated jet into a catching position whenever the observed light level from the document is above some predetermined threshold.
This invention also has application to a spirally interlacing printer of the type disclosed in Fox U.S. Pat. No. 4,069,486, which may include a plurality of rows of nozzles all spirally interlacing under control of data signals produced by an appropriately configured data system. Neither the Fox patent or Paranjpe et al teach any method for operating such spirally interlacing rows of jets to reproduce gray levels appearing on the original document.
Other techniques for controlling an ink jet printer to reproduce gray tones are disclosed in Loughren U.S. Pat. No. RE27,555, Loughren U.S. Pat. No. 3,476,874, Chen U.S. Pat. No. 3,846,800, Sagae et al U.S. Pat. No. 3,928,718, Berry U.S. Pat. No. 4,065,773, and in Hertz et al U.S. Pat. No. 3,416,153. These latter patents generally relate to methods for controlling the rate at which ink is deposited at a given location on a recording medium.